Atypical Scrapie

Jun 25

June 2009 Scrapie Canada Update

Two new cases of atypical scrapie have been confirmed by the Canadian Food Inspection Agency (CFIA) making this Canada’s second and third positive cases to date. Atypical scrapie is a newly recognized form of transmissible spongiform encephalopathy (TSE) that differs from classical scrapie, affecting a different part of the brain and producing different reactions on certain biochemical laboratory tests. Most cases are detected in much older, sometimes apparently healthy sheep of genotypes associated with increased resistance to classical scrapie. Considerable more research is required as current knowledge of this disease is limited.

The United Kingdom Spongiform Encephalopathy Advisory Committee (SEAC) has concluded that atypical scrapie can be distinguished from classical scrapie. In fact, the SEAC states that it may be more appropriate to consider atypical scrapie a distinct TSE of small ruminants, rather than a variation of the classical disease. The World Organization for Animal Health (OIE) agrees. The latest OIE Scrapie Chapter says “atypical scrapie is clinically, pathologically, biochemically and epidemiologically unrelated to ‘classical’ scrapie, may not be contagious and in effect, may be a spontaneous degenerative condition of older sheep.”

One of the first reported cases of atypical scrapie occurred in sheep in Norway in 1998. Affected sheep displayed the loss of muscle coordination in the absence of itching and scratching (a common symptom of classical scrapie in Europe).

Three cases of atypical scrapie diagnosed in Great Britain provided further information on the disease. The positive sheep came from a research flock belonging to the UK Department for Environment, Food and Rural Affairs (DEFRA). Made up of sheep (and offspring) imported from New Zealand- a country that is scrapie free- and under strict bio-security, the flock was considered to be at extremely low risk for scrapie.

Post-mortem examinations of three sheep from the UK research flock (in 2006 and 2007) found evidence of atypical scrapie. A research report into the three cases published in the BMC Veterinary Research Journal in February 2009 speculates that it is likely that the atypical scrapie arose spontaneously in the three sheep and did not come from an external source. The report also hypothesizes that atypical scrapie is likely to occur in flocks worldwide, especially in older sheep of susceptible genotypes. The three sheep in question were all over six years old. To date, no evidence of atypical scrapie has been found in New Zealand and the country still maintains its scrapie-free status.

Over the past decade, similar atypical scrapie cases have been found at a low level in many other areas including most countries in the European Union. Portugal has had an unusually high number of cases of atypical scrapie; Canada has detected two cases of atypical scrapie since 2005; and in the past two to three years, the US has detected six cases of atypical scrapie.

The surveillance rates in North America appear sufficient to detect the presence of atypical scrapie. The current test that the CFIA uses for surveillance sampling is recognized by the OIE as capable of detecting all strains of scrapie, as well as other TSEs. As such, Canada anticipates detecting sporadic cases of atypical scrapie. When this happens, additional laboratory characterization of the strain of scrapie and the genotype of the affected sheep will be undertaken in order to learn as much as possible from the case.

In recognition of the general international understanding that atypical scrapie is probably not a contagious disease, and occurs frequently as a single case within a flock, the CFIA will not respond to a case of atypical scrapie in the same manner as a case of classical scrapie. Canadian sheep flocks in which a case of atypical scrapie is detected will be screened for the presence of concurrent classical scrapie. If classical scrapie is not detected, it is highly unlikely that CFIA will take any additional measures related to the detection of the case of atypical scrapie.

When it comes to human health, there is no basis to suggest that atypical scrapie poses a risk to humans. Thus far, scientists have found no evidence that atypical scrapie can be transmitted to people, or that it is dangerous to people in any way.

Finally, the confirmation of atypical scrapie cases has not appeared to have had an impact on international trade and associated requirements. The first report of the detection of a case of atypical scrapie in a country recognized as scrapie-free (eg. New Zealand or Australia) may be different, but to date this has not happened.